The breach was restricted only to Target's physical stores, so online shoppers are (ironically) safe in this case, but anyone who shopped in a Target store between the week of Black Friday (November 27th) through until the breach was discovered and sealed on December 15th may have had their information stolen.

You should remain vigilant for incidents of fraud and identity theft by
regularly reviewing your account statements and monitoring free credit
reports. If you discover any suspicious or unusual activity on your
accounts or suspect fraud, be sure to report it immediately to your
financial institutions. In addition, you may contact the Federal Trade
Commission ("FTC") or law enforcement to report incidents of identity
theft or to learn about steps you can take to protect yourself from
identity theft.

The statement goes on to recommend that you check your credit report periodically from each of the three major credit bureaus, which you can check for free once a year at AnnualCreditReport.com.

Target has said they alerted financial institutions and agencies immediately after confirming the breach, so ideally if your card was compromised, you should hear from your bank. While there have been no reports of stolen credit cards from the breach actually being used just yet, it's best to keep an eye on your account statements anyway—sometimes fraudsters will process small charges to see if you're looking before ringing up big purchases.